Convert all 3-digit hex codes to 6-digits

[devend711]

See this issue: https://github.com/premailer/css_parser/issues/94

Any rule like:

.my-content { color: #abc; }

should be added to the rule set with the 6-character equivalent of the hex code (#aabbcc)

https://www.w3.org/TR/css-color-3/